An atom has a small, positively charged nucleus at its centre. The nucleus is made of protons and neutrons. Around it, negatively charged electrons move in the space outside the nucleus.
The nucleus is tiny. Its radius is much smaller than the radius of the whole atom, so the atom is mostly empty space. The electrons take up the space, but they have almost no mass.
Protons and neutrons are far heavier than electrons, so almost all of the mass of an atom is in the nucleus. An atom has no overall charge, because the number of positive protons equals the number of negative electrons.
